Variable pitch propeller systems typically incorporate a plurality of propeller blades mounted to a rotary hub which is driven by the aircraft engine. Each propeller blade extends radially outwardly from the hub along a longitudinal axis of the blade. In order to permit pitch adjustment, each blade is mounted to the hub for pivotable movement about its longitudinal axis. The hub typically encloses a chamber within its interior wherein a pitch change actuation system is disposed in operative association with the propeller blades. The actuation system functions to selectively change the pitch of the blades thereby altering air resistance to the rotation of the blades to control engine speed.
In most modern aircraft, the pitch change actuator is of the hydromechanical type wherein an output member, typically a piston, is driven in response to adjustments in the pressure of hydraulic fluid which drives the actuator. Adjustments in fluid pressure are typically affected by either a hydromechanical or electronic control system which monitors engine speed and causes, by an associated apparatus, a change in fluid pressure whenever the monitored engine speed departs from the desired engine speed setting.
Typical propeller systems also include a plurality of rotating parts and a plurality of stationary parts, wherein the stationary parts generally include the control elements of the system. Accordingly, an electronic engine control (EEC) and a protection valve are usually provided on the non-rotational portion of the system wherein the protection valve under control by the EEC ports hydraulic fluid to the pitch actuation assembly through a transfer bearing. In typical prior art systems, translating transfer tubes are used to direct the hydraulic fluid from the transfer bearing to the pitch actuation mechanism. Accordingly, and as shown in FIG. 4, current propeller systems provide access to the actuation systems through the centerline of the propeller shaft via the translating transfer tubes. The translating transfer tubes extend from the front of the actuator to the fwd portion of the control unit. The translating transfer tubes are connected to a piston/yoke assembly, whereby the yoke engages a cam at the base of the propeller blade and the piston assembly is translated via the porting of hydraulic fluid from the translating transfer tube to the increase pitch and a decrease pitch side thereof.
As shown in the FIG. 4, by monitoring the movement of the translating transfer tube, the pitch angle of the propeller blades can be determined via a blade angle feedback mechanism. If a minimum blade angle is detected, a secondary low pitch solenoid can be actuated for preventing further movement of the propeller blades towards lower pitch, thereby preventing dangerous excursions toward low pitch while the aircraft is in flight. As is known in the art, operation at high power and low pitch angle conditions could result in severe engine and propeller overspeed conditions and certain in-flight emergency conditions.
In addition to the secondary low pitch stop, most systems also include an overspeed governor which senses the speed of the aircraft propellers and upon sensing overspeed, also functions to prevent further pitch change towards low pitch. The overspeed governor, blade angle feedback mechanism and secondary low pressure stop of the prior art systems are typically located on the non-rotating, control portion of the propeller systems.
One disadvantage of prior art systems arises during disassembly of the control portion of the propeller actuator, which typically requires removal of the transfer tube assembly and spinner. Since the male portion of the oil transfer bearing is integral with the end of the transfer tube, handling of the transfer tube by mechanics and the like may and often does result is damage to the bearing surface. Damaged bearing surfaces can ultimately result in bearing seizure.
Further, the axial location of the translating transfer tube must be rigged properly at installation in order to set the secondary low pitch stop blade angle and provide blade angle feedback, as these functions are critical to control the aircraft. In current systems, the transfer tube is classified as a flight safety part because failure in performance thereof with regard to these two functions could compromise the safe operation of aircraft. Accordingly, the current design relies too heavily on the flight mechanic to install the transfer tube correctly to ensure that the propeller control functions as intended.
There exists a need therefor, for a simplified design of the elements of the pitch change actuation mechanism of current propeller systems, including the use of a non-translating transfer tube, and a secondary low pitch stop and overspeed governor which are separate in function and in structure from the transfer tube and which are located on the rotating portion of the propeller for simplifying their design and implementation.